Light emitting diode and method for fabricating the same

ABSTRACT

A light emitting diode and a method for fabricating the same are provided. The light emitting diode includes: a transparent substrate; a semiconductor material layer formed on the top surface of a substrate with an active layer generating light; and a fluorescent layer formed on the back surface of the substrate with controlled varied thicknesses. The ratio of light whose wavelength is shifted while propagating through the fluorescent layer and the original light generated in the active layer can be controlled by adjusting the thickness of the fluorescent layer, to emit desirable homogeneous white light from the light emitting diode.

BACKGROUND OF THE INVENTION

This application claims priority from Korean Patent Application No.2002-52462, filed on Sep. 2, 2002, in the Korean Intellectual PropertyOffice, the disclosure of which is incorporated herein in its entiretyby reference.

1. Field of the Invention

The present invention relates to a light emitting diode and a method forfabricating the same, and more particularly, to a light emitting diodecapable of emitting light with a homogeneous color profile and a methodfor fabricating the same.

2. Description of the Related Art

FIG. 1 is a sectional view of a conventional light emitting diodeemitting white light. Referring to FIG. 1, a blue-light emitting diode 2is located inside a lead frame 1, with wires 3 connected to the topsurface thereof via ohmic contact and to the lead frame 1 to supplyelectricity. The inner space of the lead frame 1 is filled with aphosphor 6 so that blue light emitted from the blue-light emitting diode2 is converted to red or green light or and then to white light by beingmixed up with the red or green light.

However, such a conventional blue-light emitting diode 2 cannot emithomogeneous white light profile and tends to emit light of wavelengthsdifferent from the wavelength of white light or conical light with ayellow or blue light ring.

Various solutions to this problem have been suggested. For example,according to a light emitting diode disposed in U.S. Pat. No. 5,813,753,as shown in FIG. 2, a light emitting diode 11 is provided in acup-shaped header 12. A mirror 13 is formed on the inner wall of theheader 12 to reflect light emitted from the light emitting diode 11. Theinner space of the header 12 is filled with a transparent material 15containing phosphor grains 14 dispersed around the light emitting diode11. A glass plate 16 is placed on the top of the header 12 to preventlight which is not absorbed by the phosphor grains 14 from being emittedinto the air. A low-wave pass (LWP) filter is further placed on thefront side of the light emitting diode 11 to pass short-wavelength lightmore efficiently than long-wavelength light.

However, in manufacturing such conventional light emitting diodes, it isdifficult to control the amount of phosphor grains that is necessary toemit light of desired wavelength bands. A transparent materialcontaining phosphor grains should be deposited for individual lightemitting diodes. Accordingly, a great chromatic difference between theseparate light emitting diodes occurs, and the manufacturing timeincreases.

EP 0 855 751 A2 discloses an organic/inorganic semiconductor lightemitting diode emitting red light and blue light that is manufactured byappropriately doping a green phosphor layer. However, it is difficult touniformly dope the phosphor layer to an appropriate ion concentration toobtain light of a uniform color profile.

SUMMARY OF THE INVENTION

The present invention provides a light emitting diode with a fluorescentlayer having controlled varied thicknesses, wherein the thickness of thefluorescent layer can be appropriately adjusted to enable the lightemitting diode to emit light of a desired wavelength band, and a simplemethod for fabricating the same.

According to an aspect of the present invention, there is provided alight emitting diode comprising: a substrate which transmits light; asemiconductor material layer formed on the top surface of a substratewith an active layer generating light; and a fluorescent layer formed onthe back surface of the substrate with controlled varied thicknesses.The substrate may have at least one etched hole formed by etching theback surface of the substrate to controlled varied thicknesses. Thefluorescent layer may be formed as dual layers with controlled variedthicknesses. It is preferable that the substrate is a sapphiresubstrate.

In an embodiment of the light emitting diode according to the presentinvention, the semiconductor material layer may comprise: a firstcompound semiconductor layer deposited on the top surface of thesubstrate; the active layer deposited on the top surface of the firstcompound semiconductor layer; and a second compound semiconductor layerdeposited on the top surface of the active layer. In this case, thefirst compound semiconductor layer may be an n-type doped or undopedGaN-based III-V nitride compound semiconductor layer. The secondcompound semiconductor layer may be a p-type doped GaN-based III-Vnitride compound semiconductor layer. The active layer may be an n-typedoped or undoped In_(x)Al_(y)Ga_(1−x−y)N compound semiconductor layerwhere 0≦x≦1, 0≦y≦1, and

In a light emitting diode according to the present invention, the activelayer generates blue light, and the fluorescent layer converts a portionof the blue light to yellow light to emit white light from the lightemitting diode. In this case, the fluorescent layer may be formed of afluorescent material including a garnet fluorescent material activatedwith cerium containing at least one element selected from the groupconsisting of yttrium, lutetium, scandium, lanthanum, gadolinium, andsamarium, and at least one element selected from the group consisting ofaluminum, gallium, and indium.

Alternatively, the active layer may generate UV light, and thefluorescent layer may convert the UV light to red, green, and blue lightby absorbing the UV light, to emit white light from the light emittingdiode. In this case, the fluorescent layer may be formed of afluorescent material containing a red phosphor selected from the groupconsisting of Y₂O₃Eu³⁺Bi³⁺ and Y₂O₂S, a green phosphor selected from thegroup consisting of(Ba_(1−x−y−z)Ca_(x)Sr_(y)Eu_(z))(Mg_(1−w)Zn_(w))Si₂O₇ and ZnS:Cu, and ablue phosphor selected from the group consisting of (Sr, Ba,Ca)₅(PO₄)₃Cl:Eu²⁺) (SECA), BaMg₂Al₁₆O₂₇:Eu²⁺ (BAM), and BaMgAl₁₀O₁₇:Eu.

The present invention provides a light emitting diode with a fluorescentlayer having controlled varied thicknesses that can be implemented byetching the back surface of a substrate or by depositing a fluorescentmaterial on the back surface to controlled varied thicknesses. Accordingto the present invention, the emission ratio of original blue lightgenerated in an active layer and light absorbed by the fluorescent layerand converted to yellow light from the blue light can be controlled byappropriately adjusting the thickness of the fluorescent layer, to emithomogeneous white light from the light emitting diode. When the activelayer generates UV light, the emission ratio of the original UV lightand light absorbed by the fluorescent layer and converted to red, green,and blue light from the UV light can be controlled to emit homogeneouswhite light from the light emitting diode.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a sectional view of a conventional white-light emitting diode;

FIG. 2 is a sectional view of a light emitting diode disclosed in U.S.Pat. No. 5,813,753;

FIG. 3 is a sectional view of a light emitting diode according to afirst embodiment of the present invention;

FIG. 4 is a sectional view of a light emitting diode according to asecond embodiment of the present invention;

FIG. 5 is a sectional view of a light emitting diode according to athird embodiment of the present invention;

FIG. 6 is a sectional view of a light emitting diode according to afourth embodiment of the present invention;

FIGS. 7A through 7D are sectional views illustrating a first embodimentof a method for manufacturing light emitting diodes according to thepresent invention;

FIGS. 8A through 8E are sectional views illustrating a second embodimentof the method for manufacturing light emitting diodes according to thepresent invention;

FIGS. 9A and 9B are photographs showing etched substrates of lightemitting diodes according to the first embodiment of the presentinvention; and

FIGS. 10A and 10B are photographs showing the back surface of thesubstrates of FIGS. 9A and 9B, respectively, with fluorescent layers.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of a light emitting diode and a method for fabricating thesame will be described in detail.

Referring to FIG. 3, which is a sectional view showing the structure ofa light emitting diode according to an embodiment of the presentinvention, a light emitting diode 50 includes a substrate 51, and afirst compound semiconductor layer 53, an active layer 57, and a secondcompound semiconductor layer 55, which are sequentially deposited on thetop surface of the substrate 51. An n-type electrode 54 is placed in astepped region of the first compound semiconductor layer 53, and ap-type electrode 52 is placed on the top surface of the second compoundsemiconductor layer 55, to supply electrons and holes into the activelayer 57.

The substrate 51 is made of a durable substance, mostly such assapphire. The back surface of the substrate 51 is etched to reduce thethickness of the substrate 51 in certain areas. Here, it is preferableto form an etched hole 56 a in the back surface of the substrate 51 byetching. In this case, the thickness of the substrate 51 is larger at aperipheral region 56 b than at the etched hole 56 a. The differentthicknesses of the substrate 51 enable a fluorescent layer 59 to bedeposited to controlled varied thicknesses onto the back surface of thesubstrate 51 by spin coating.

The first compound semiconductor layer 53 is a GaN-based III-V nitridesemiconductor layer, and preferably, is a direct transition type. In thecase of doping the first compound semiconductor with conductiveimpurities, a GaN layer is preferable for the first compoundsemiconductor layer 53. In either case, it is preferable that the firstcompound semiconductor layer 53 is formed of the same maternal as thesecond compound semiconductor layer 55. A first cladding layer (notshown) may be further formed on the top surface of the first compoundsemiconductor layer 53. Preferably, the first cladding layer may beformed of an n-AlGaN/GaN layer having a predetermined refractive index.However, the first cladding layer may be formed of a compoundsemiconductor layer different from the n-AlGaN/GaN layer.

The second compound semiconductor layer 55 is a GaN-based III-V nitridesemiconductor layer, and preferably, is a direct transition type dopedwith p-type conductive impurities, and most preferably, is a p-GaNlayer. In the case of undoping the second compound semiconductor layer55, a GaN layer or a AlGaN layer or InGaN layer containing Al or In,respectively, in a predetermined ratio may be used for the secondcompound semiconductor layer 55.

The active layer 57 is formed on the top surface of the first compoundsemiconductor layer 53. The active layer 57 is a material layer wherelight is generated by the recombination of electrons and carrier holes.Preferably, the active layer 57 is a GaN-based III-V nitridesemiconductor layer with a multiple quantum-well (MOW) structure. Morepreferably, the active layer 57 is formed of a In_(x)Al_(y)Ga_(1−x−y)N,where 0≦x≦1, 0≦y≦1, and x+y≦1, with a MQW structure.

First and second waveguide layers (not shown) may be further formed onand underneath the active layer 57, respectively, to amplify lightemitted from the active layer 57 and emit light from the LED withenhanced intensity. The first and second waveguide layers are formed ofa smaller refractive index material than the active layer 57, andpreferably, for example, a GaN-based III-V compound semiconductor layer.The first waveguide layer may be formed of a n-GaN layer, and the secondwaveguide layer may be formed of a p-GaN layer. The active layer 57 maybe formed of any material having a small threshold current value andstable transverse mode properties. Preferably, the active layer 57 isformed of an AlGaN layer containing Al in a predetermined ratio.

The second compound semiconductor layer 55 is formed on the top surfaceof the active layer 57. A second cladding layer (not shown) having asmaller refractive index than the second waveguide layer may beadditionally formed between the second compound semiconductor layer 55and the active layer 57. This second cladding layer is formed of ap-type compound semiconductor layer when the first cladding layer isformed of a n-type compound semiconductor layer, and is formed of ap-type compound semiconductor layer when the first cladding layer isformed of a p-type compound semiconductor layer. For example, when thefirst cladding layer is formed of a n-AlGaN/GaN layer, the secondcladding layer is formed of a p-AlGaN/GaN layer.

A pair of n-type electrodes 54 are laid on the two stepped regions ofthe first compound semiconductor layer 53, and the p-type electrode 52is laid on the top surface of the second compound semiconductor layer55, via which electrons and holes are injected into the first compoundsemiconductor layer 53 and the second compound semiconductor layer 55,respectively. The injected electrons and holes combine together anddisappear in the active layer 57 to oscillate light of ashort-wavelength band. The color of emitted light varies depending onthe wavelength band. The wavelength band of light is determined by theenergy width between the conduction band and valence band of thematerial used to form the light emitting diode 50.

III-V nitrides are commonly used to form semiconductor material layersemitting blue, green, and UV light. In the present invention,specifically, GaN-based semiconductor materials among III-V nitrides areused to enable the active layer 57 to generate blue light of awavelength of 420-470 nm or UV light and the generated blue light to betransmitted through a fluorescent layer 59 deposited on the back surfaceof the substrate 51. A portion of the generated blue light is absorbedin the fluorescent layer 59 and emitted as light of a differentwavelength band from the original blue light, for example, yellow light,and the non-absorbed blue light is emitted as blue light having theoriginal wavelength.

Various kinds of fluorescent materials may be selectively used dependingon the wavelength band of desired light to emit. When a light emittingdiode is formed of a nitride semiconductor material emitting blue light,as a fluorescent material capable of converting the blue light to yellowlight, a garnet fluorescent material activated with cerium (Ce)including at least one element selected from the group consisting ofyttrium (Y), lutetium (Lu), scandium (Sc), lanthanum (La), gadolinium(Gd), and samarium (Sm), and at least one element selected from thegroup consisting of aluminum (Al), gallium (Ga), and indium (In) may beused. To control the wavelength of emitted light, in a mixture of Ygroup, Al group, and garnet fluorescent materials, two kinds offluorescent materials selected from the Y group may be used together innon-equal amounts. For example, a portion of Y may be substituted by Gd.

In a light emitting diode with an active layer emitting blue light of awavelength of 420-470 nm, suitable fluorescent materials capable ofconverting the blue light to red light of a wavelength of 610-625 nminclude Y₂O₂S:Eu³⁺,Bi⁺; YVO₄:Eu³⁺,Bi³⁺; SrS:Eu²⁺; SrY₂S₄:Eu²⁺;CaLa₂S₄:Ce³⁺; (Ca, Sr)S:Eu²⁺ and the like. Suitable fluorescentmaterials capable of converting the blue light to green light of awavelength of 530-555 nm include YBO₃:Ce³⁺,Tb³⁺; BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺;(Sr,Ca,Ba)(Al,Ga)₂S₄:Eu²⁺ and the like. Any fluorescent materialemitting red light or green light may be used.

When a light emitting diode is formed of a nitride semiconductormaterial emitting UV light, a fluorescent material containing a redphosphor, such as Y₂O₃Eu³⁺Bi³⁺ and Y₂O₂S, a green phosphor, such as(Ba_(1−x−y−z)Ca_(x)Sr_(y)Eu_(z))(Mg_(1−w)Zn_(w))Si₂O₇ and ZnS:Cu, and ablue phosphor, such as (Sr, Ba,Ca)₅(PO₄)₃Cl:Eu²⁺) (SECA),BaMg₂Al₁₆O₂₇:Eu²⁺ (BAM), and BaMgAl₁₀O₁₇:Eu, is used for the fluorescentlayer 59 formed on the etched back surface of the substrate havingcontrolled varied thicknesses. In this case, UV light generated in theactive layer 57 is converted to red, green, and blue light whilepropagating through the fluorescent layer 59 and finally emitted fromthe light emitting diode as white light.

In the present invention, as shown in FIG. 3, since the back surface ofthe substrate 51 has the etched hole 56 a, which is filled with thefluorescent layer 56, the thickness of the fluorescent layer 59 islarger at the etched hole 56 a than at the peripheral region 56 b. As aresult, light generated in the active layer 57 is absorbed more by thefluorescent material while propagating through the thicker region of thefluorescent layer 59 a than through the thinner region corresponding tothe peripheral region 56 b so that a larger amount of light whosewavelength band is shifted compared to the original light is emitted.

While the wavelength of emitted light is controlled using fluorescentmaterials in conventional light emitting diodes, the thickness of thefluorescent layer 5 appropriately varied in the present invention inorder to emit light of a desired wavelength band. Alternatively,luminescence can be enhanced by changing the shape of the etched hole 56a of the substrate 51. For example, the sloping angle and the bottomcurvature of the etched hole 56 a may be varied in order to control theamount of light incident on the fluorescent layer 59 through thesubstrate.

FIG. 4 is a sectional view of a light emitting diode according to asecond embodiment of the present invention with a plurality of etchedholes.

Referring to FIG. 4, the back surface of the substrate 61 is etched toform a plurality of etched holes 66 a, and a fluorescent layer 69 isformed to fill over the etched holes 66 a in the substrate 61, so thatthe structure of a light emitting diode as shown in FIG. 4 is obtained.

Blue light or UV light generated in an active layer 67 of asemiconductor material layer 65 is transmitted through the substrate 62and enters the fluorescent layer 69. Since the fluorescent layer 69 hasa larger thickness at the etched holes 66 a than at peripheral regions66 b, light incident on the etched holes 66 a and propagating throughthe thicker region of the fluorescent layer 69 is likely to excite andabsorb more fluorescent grains present in the fluorescent layer 69,compared with light propagating through the peripheral regions 66 b. Inother words, the blue light or UV light generated in the active layer 67is highly likely to be converted to yellow light, or red, green and bluelight having a different wavelength from the original blue or UV lightwhile propagating through the thicker region of the fluorescent layer69, where the etched holes 66 a are formed. Also, light propagatingthrough the thinner region of the fluorescent layer 69, where theperipheral regions 66 b are formed, is highly likely to be emitted asthe original blue or UV light, without shifting in wavelength band.

The thickness of the fluorescent layer 69 can be adjusted to differentlevels by appropriately varying the number and the depth of etched holes66 a. As a result, light generated in the active region 67 of thesemiconductor material layer 65 is converted to light of wavelengthbands different from the original light while propagating through thefluorescent layer 69, so that homogeneous white light can be emittedfrom the light emitting diode.

In FIG. 4, reference numeral 62 denotes a p-type electrode, andreference numeral 64 denotes an n-type electrode. The material,properties, and function of the compound semiconductor layersconstituting the light emitting diode of FIG. 4 are the same as those ofthe light emitting diode according to the first embodiment describedabove.

FIG. 5 is a sectional view showing the structure of a light emittingdiode according to a third embodiment of the present invention.Referring to FIG. 5, a substrate 71 has a uniform thickness. A firstfluorescent layer 79 a is deposited on the back surface of the substrate71, and a second fluorescent layer 79 b is formed on a region of thefirst fluorescent layer 79 a. Accordingly, the entire fluorescent layer,including the first and second fluorescent layers 79 a and 79 b, hascontrolled varied thicknesses. Reference numeral 72 denotes a p-typeelectrode, reference numeral 74 denotes an n-type electrode, andreference numeral 75 denotes a semiconductor material layer.

FIG. 6 is a sectional view showing the structure of a light emittingdiode according to a fourth embodiment of the present invention.Referring to FIG. 6, a substrate 81 has a uniform thickness. A firstfluorescent layer 89 a is deposited on the back surface of the substrate81, and a plurality of second fluorescent layers 89 b are formed on thefirst fluorescent layer 89 a as stripes. Accordingly, the entirefluorescent layer, including the first and second fluorescent layers 89a and 89 b, has controlled varied thicknesses. Reference numeral 72denotes a p-type electrode, reference numeral 74 denotes an n-typeelectrode, and reference numeral 75 denotes a semiconductor materiallayer. Alternatively, a plurality of second fluorescent layers 89 a maybe formed as dots.

In the above-described third and fourth embodiments, the material,properties, and function of the compound semiconductor layerconstituting each of the light emitting diodes are the same as those ofthe light emitting diode according to the first embodiment of thepresent invention. The principles of emitting white light using thefluorescent layers 79 a (89 a) and 79 b (89 b) having controlled variedthicknesses are similar to those as in the first embodiment. Althoughthe fluorescent layer having controlled varied thicknesses isimplemented by etching the substrate in the light emitting diodesaccording to the first and second embodiments of the present invention,in the light emitting diodes according to the third and fourthembodiments of the present invention, the fluorescent layer havingcontrolled varied thicknesses is implemented using two separatefluorescent layers 79 a (89 a) and 79 b (89 b).

In the light emitting diodes according to the third and fourthembodiments of the present invention, when blue or UV light generated inthe active layer 77 (87) of the semiconductor material layer 75 (85)propagates through both of the first and second fluorescent layers 79 a(89 a) and 79 b (89 b), the blue or UV light is highly likely to beshifted in wavelength band and emitted as yellow light or red, green,and blue light, compared with blue or UV light propagating only throughthe first fluorescent layer 79 a (89 a). In other words, it is possibleto generate homogeneous white light by appropriately varying thethickness and the number of patterns constituting the second fluorescentlayer 79 b (89 b).

The light emitting diodes according to the first through fourthembodiments of the present invention described above are forillustrative purposes and, therefore, the shape and number of etchedholes and the thickness and the shape of the fluorescent layer may bevariously changed.

FIGS. 7A through 7D are sectional views illustrating a first embodimentof a method for fabricating light emitting diodes according to thepresent invention. Referring to FIG. 7A, a first compound semiconductorlayer 53, an active layer 57, and a second compound semiconductor layer55 are deposited in sequence on the top surface of a substrate 51, andthe first compound semiconductor layer 53 is patterned byphotolithography to form a step in the first compound semiconductorlayer 53. N-type electrodes 54 are laid on the patterned surface of thefirst compound semiconductor layer 53 as stripes, and p-type electrodes52 are laid on the top surface of the second compound semiconductorlayer 55.

Referring to FIG. 7B, etched holes 56 b, which are filled with afluorescent layer 57 later, are formed in the back surface of thesubstrate 51 by dry etching. Prior to etching the substrate 51, the backsurface of the substrate 51 is processed by grinding, lapping, orpolishing. A mask layer (not shown) is formed on the back surface of thesubstrate 51 and patterned into a mask pattern corresponding to theetched holes 56 a. The back surface of the substrate 51 is etched usingat least one gas selected from the group consisting of Cl₂, BCl₃, Ar,O₂, and HBr, with the mask pattern serving as an etch mask, so that theetched holes 56 are formed as shown in FIG. 7B.

FIGS. 9A and 9B show etched holes formed in sapphire substrates byetching. Etched holes 56 c of FIG. 9A are relatively wide and shallow,and etched holes 56 d of FIG. 9B are relatively narrow and deep. Thedepth and width of the etched holes 56 c (56 d) are determined inconsideration of the thickness of the fluorescent layer depositedtherein. For example, the back surface of a sapphire substrate is etchedsuch that the resulting etched holes 56 c (56 d) have a depth of about50 μm and a width of 250-500 μm.

Referring to FIG. 7C, after the etched holes 56 a have been formed, afluorescent material is applied to the etched holes 56 a and theperipheral regions 56 b by disposing or spin coating to form thefluorescent layer 59, as shown in FIG. 6C. As a result, a light emittingdiode structure 58 is formed. According to the present invention, thefluorescent layer 59 can be uniformly deposited over the entiresubstrate 51 through a single process. Therefore, the overall processfor fabricating light emitting layer diodes is simple.

FIGS. 10A and 10B are photographs of light emitting diodes withyttrium-aluminum-garnet (YAG) fluorescent layers 59 c and 59 d formed byapplying a fluorescent material to fill the etched holes 56 c and 56 din the back surface of the substrates of FIGS. 9A and 9B, respectively.In FIGS. 10A and 10B, the YAG fluorescent layers 59 c and 59 d in theetched holes appear white.

Finally, as shown in FIG. 7D, the light emitting diode structure 58 iscut, at connection regions with two adjacent n-type electrodes 54, intoa plurality of light emitting diodes 50.

FIGS. 8A through 8E are sectional views illustrating a second embodimentof the method for fabricating light emitting diodes according to anembodiment of the present invention. Referring to FIG. 8A, a firstcompound semiconductor layer 73, an active layer 77, and a secondcompound semiconductor layer 75 are deposited in sequence on the topsurface of a substrate 71, and the first compound semiconductor layer 73is patterned by photolithography to form a step in the first compoundsemiconductor layer 73. N-type electrodes 74 are laid on the patternedsurface of the first compound semiconductor layer 73 as stripes, andp-type electrodes 72 are laid on the top surface of the second compoundsemiconductor layer 75.

Referring to FIG. 8B, a fluorescent material is applied to the backsurface of the substrate 71 by disposing or spin coating to form a firstfluorescent layer 79 a. Next, as shown in FIG. 8C, a mask 76 having apredetermined pattern is placed on the first fluorescent layer 79 a, andthe fluorescent material is applied to form a a second fluorescent layer79 b on the first fluorescent layer 79 a. As a result, a light emittingdiode structure 78 with a fluorescent layer, including the first andsecond fluorescent layers 79 a and 79 b, having controlled variedthicknesses is formed, as shown in FIG. 8D. Finally, as shown in FIG.8E, the light emitting diode structure 78 is cut into a plurality oflight emitting diodes 50, thereby completing the fabrication of desiredlight emitting diodes 50.

A light emitting diode according to the present invention is fabricatedwith a fluorescent layer having controlled varied thicknesses, whereinthe fluorescent layer having controlled varied thicknesses may be formedby etching a substrate to controlled varied thicknesses and applying afluorescent material to the etched surface of the substrate.Alternatively, the fluorescent layer having controlled variedthicknesses may be formed by depositing a fluorescent material tocontrolled varied thicknesses on a substrate having a uniform thickness.In the light emitting diode according to the present invention, lightgenerated in an active layer is shifted in wavelength while passingthrough the fluorescent layer having controlled varied thicknesses. Theratio of light whose wavelength band is shifted while propagatingthrough the fluorescent layer and the original light generated in theactive layer can be controlled by varying the thickness of thefluorescent layer so that desired homogeneous white light can be emittedfrom the light emitting diode according to the present invention. Amethod for fabricating light emitting diodes according to the presentinvention, which involves simple processes, for example, etching theback surface of a substrate or applying a fluorescent material over thesubstrate by disposing or spin coating, is suitable for mass production.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1-37. (canceled)
 38. A light emitting diode comprising: a lighttransmitting substrate having a top surface, a back surface opposite tothe top surface and a side surface between the top surface and the backsurface, at least one recess being disposed on the back surface; asemiconductor material layer having a first compound semiconductorlayer, an active layer generating a first light and a second compoundsemiconductor layer disposed on the top surface of the substrate in thatorder, the semiconductor material layer emitting the first light via theback surface of the substrate; at least one first-type electrodeconnected to the first compound semiconductor layer and at least onesecond-type electrode connected to the second compound semiconductorlayer, respectively; and a wavelength converting layer disposed on theback surface of the substrate and converting a portion of the firstlight to a second light having a different wavelength from the firstlight, wherein the area of the back surface of the substrate adjacentthe recess is substantially flat.
 39. The light emitting diode of claim38, wherein the at least one recess has a sloped sidewall.
 40. The lightemitting diode of claim 39, wherein the sloped sidewall is inclined suchthat the recess has a larger width at the top of the recess than a widthat the bottom of the recess.
 41. The light emitting diode of claim 38,wherein the wavelength converting layer covers the entire back surfaceof the substrate.
 42. The light emitting diode of claim 38, wherein thewavelength converting layer fills the entirety of the at least onerecess.
 43. The light emitting diode of claim 38, wherein the wavelengthconverting layer is disposed on an area of the back surface where therecess is not disposed.
 44. The light emitting diode of claim 38,wherein the wavelength converting layer has varied thicknesses along adirection in parallel with the top surface of the substrate.
 45. Thelight emitting diode of claim 38, wherein the first compoundsemiconductor layer has a first portion and a second portion having ahigher thickness than that of the first portion, the thicknesses of thefirst portion and the second portion being thicknesses of the firstcompound semiconductor layer in a direction perpendicular the topsurface of the substrate.
 46. The light emitting diode of claim 38,wherein the first compound semiconductor layer has a first portion and asecond portion, and wherein the active layer and the second compoundsemiconductor layer are disposed on the second portion of the firstcompound semiconductor layer in that order, and the at least onefirst-type electrode is disposed on the first portion of the firstcompound semiconductor layer.
 47. The light emitting diode of claim 45,wherein the first compound semiconductor layer has a plurality of firstportions.
 48. The light emitting diode of claim 46, wherein the at leastsecond-type electrode is disposed on the second compound semiconductorlayer, and a surface of the at least one second-type electrode oppositeto the surface facing with the second compound semiconductor layer issubstantially flat.
 49. The light emitting diode of claim 38, whereinthe first light is blue light and the second light is yellow light. 50.The light emitting diode of claim 38, wherein the wavelength convertinglayer is formed of a fluorescent material including a garnet fluorescentmaterial activated with cerium containing at least one element selectedfrom the group consisting of yttrium, lutetium, scandium, lanthanum,gadolinium, and samarium, and at least one element selected from thegroup consisting of aluminum, gallium, and indium.
 51. The lightemitting diode of claim 38, wherein the first light is blue light of awavelength of 420-470 nm, and the second light includes at least onefluorescent material of Y₂O₂S:Eu³⁺,Bi³⁺, YVO₄:Eu³⁺,Bi³⁺, SrS:Eu²⁺,SrY₂S₄:Eu²⁺, CaLa₂S₄:Ce³⁺, (Ca, Sr)S:Eu²⁺ and at least one fluorescentmaterial of YBO₃:Ce³⁺, Tb³⁺, BaMgAl₁₀O₁₇:Eu²⁺,Mn²⁺, (Sr, Ba, Ca)(Al,Ga)₂S₄:Eu²⁺.
 52. The light emitting diode of claim 38, wherein the firstlight is UV light, and the second light includes red, green and bluelight.
 53. The light emitting diode of claim 38, wherein the first lightis blue light of a wavelength of 420˜470 nm, and the second light isyellow light.
 54. The light emitting diode of claim 38, wherein thefirst light is blue light of a wavelength of 420˜470 nm, and the secondlight includes red and green.
 55. The light emitting diode of claim 38,wherein the first light is blue light of a wavelength of 420˜470 nm, andthe second light includes red of a wavelength of 610˜625 nm and/or greenof a wavelength of 530˜555 nm.
 56. The light emitting diode of claim 38,wherein the substrate is a sapphire substrate.
 57. The light emittingdiode of claim 38, wherein the first compound semiconductor layer is ann-type doped or undoped GaN-based III-V nitride compound semiconductorlayer.
 58. The light emitting diode of claim 38, wherein the secondcompound semiconductor layer is a p-type doped GaN-based III-V nitridecompound semiconductor layer.
 59. The light emitting diode of claim 38,wherein the active layer is an n-type doped or undopedIn_(x)Al_(y)Ga_(z)N compound semiconductor layer where 0≦x≦1, 0≦y≦1,0≦z≦1 and 0≦x+y+z≦1.
 60. A light emitting diode comprising: a lighttransmitting substrate having a top surface, a back surface opposite tothe top surface and a side surface between the top surface and the backsurface, the substrate having at least one thick area having a firstthickness and at least one thin area having a second thickness smallerthan the first thickness, the first and second thicknesses beingthicknesses from the top surface to the bottom surface, the at least onethin area being adjacent the at least one thick area; a semiconductormaterial layer having a first compound semiconductor layer, an activelayer generating a first light and a second compound semiconductor layerdisposed on the top surface of the substrate in that order, thesemiconductor material layer emitting the first light via the backsurface of the substrate; at least one first-type electrode connected tothe first compound semiconductor layer and at least one second-typeelectrode connected to the second compound semiconductor layer,respectively; and a wavelength converting layer disposed on the backsurface of the substrate and converting a portion of the first light toa second light having a different wavelength from the first light,wherein the back surface of the substrate at the at least one thick areais substantially flat, wherein the thickness of the wavelengthconverting layer disposed on the at least one thick area is smaller thanthe thickness of the wavelength converting layer disposed on the atleast one thin area.
 61. The light emitting diode of claim 60, whereinthe at least two thick areas and the at least one thin area are disposedsuch that at least one recess is disposed at the least one thin area onthe back surface of the substrate.
 62. The light emitting diode of claim61, wherein the at least one recess has sloped sidewalls.
 63. The lightemitting diode of claim 62, wherein the at least sloped sidewall isinclined such that the at least one recess has a larger width at the topof the recess than a width at the bottom of the recess
 64. The lightemitting diode of claim 60, wherein the wavelength converting layercovers the entire back surface of the substrate.
 65. The light emittingdiode of claim 61, wherein the wavelength converting layer fills theentirety of the at least one recess.
 66. The light emitting diode ofclaim 60, wherein the first compound semiconductor layer has a firstportion and a second portion having a higher thickness than that of thefirst portion, the thicknesses of the first portion and the secondportion being thicknesses of the first compound semiconductor layer in adirection perpendicular the top surface of the substrate .
 67. The lightemitting diode of claim 60, wherein the first compound semiconductorlayer has a first portion and a second portion, and wherein the activelayer and the second compound semiconductor layer are disposed on thesecond portion of the first compound semiconductor layer in that order,and the at least one first-type electrode is disposed on the firstportion of the first compound semiconductor layer.
 68. The lightemitting diode of 67, wherein the first compound semiconductor layer hasa plurality of first portions.
 69. The light emitting diode of claim 67,wherein the at least one second-type electrode is disposed on the secondcompound semiconductor layer, and a surface of the at least onesecond-type electrode opposite to the surface facing with the secondcompound semiconductor layer is substantially flat.